When astronomers discovered the first exoplanet around a normal star
2 decades ago, there was joy - and bewilderment.

The planet,
51 Pegasi b, was half as massive as
Jupiter, but its 4-day orbit was impossibly close to the star, far
smaller than the 88-day orbit of Mercury. Theorists who study planet
formation could see no way for a planet that big to grow in such
tight confines around a newborn star.

It could have been a freak, but soon,
more "hot
Jupiters" turned up in planet searches, and they were
joined by other oddities:

planets in elongated and highly
tilted orbits, even planets orbiting their stars "backward" -
counter to the star's rotation.

The planet hunt accelerated with the
launch of NASA's Kepler spacecraft in 2009, and the 2500 worlds it
has discovered added statistical heft to the study
of
exoplanets - and yet more confusion.

Kepler found that the most common type
of planet in the galaxy is something between the size of Earth and
Neptune - a "super-Earth," which has no parallel in our solar system
and was thought to be almost impossible to make.

Now, ground-based telescopes are
gathering light directly from exoplanets, rather than detecting
their presence indirectly as Kepler does, and they, too, are turning
up anomalies.

They have found giant planets several
times the mass of Jupiter, orbiting their star at more than twice
the distance Neptune is from the sun - another region where
theorists thought it was impossible to grow large planets.

Other planetary systems looked nothing
like our orderly solar system, challenging the well-worn theories
that had been developed to explain it.

"It's been really obvious things
didn't fit pretty much from day one," says Bruce Macintosh, a
physicist at Stanford University in Palo Alto, California.

"There has never been a moment when
theory has caught up with observations."

Theorists are trying to catch up -
coming up with scenarios for growing previously forbidden kinds of
planets, in places once thought off-limits.

They are envisioning how planets could
form in much more mobile and chaotic environments than they ever
pictured before, where nascent planets drift from wide to narrow
orbits or get ricocheted into elongated or off-kilter paths by other
planets or passing stars.

But the ever-expanding zoo of exotic
planets that observers are tallying means every new model is
provisional.

"You can discover something new
every day," says astrophysicist Thomas Henning of the Max Planck
Institute for Astronomy in Heidelberg, Germany.

"It's a Gold Rush situation."

THE TRADITIONAL MODEL

THE TRADITIONAL MODEL of how stars and
their planets form dates back to the 18th century,
when scientists proposed that a slowly rotating cloud of dust and
gas could collapse under its own gravity.

Most of the material forms a ball that
ignites into a star when its core gets dense and hot enough. Gravity
and angular momentum herd the leftover material around the
proto-star into a flat disk. Dust is key to transforming this disk
into a set of planets.

The dust, which accounts for a small
fraction of the disk's mass, is made up of microscopic specks of
iron and other solids. As they swirl in the roiling disk, the specks
occasionally collide and stick together by electromagnetic forces.

Over a few million years, the dust
builds up into grains, pebbles, boulders, and, eventually,
kilometer-wide
planetesimals.

At that point gravity takes over, pulling in other planetesimals and
vacuuming up dust and gas until planet-sized bodies take shape.

By the time that happens in the inner
part of the disk, most of its gas has been stripped away, either
gobbled up by the star or blown away by its
stellar wind. The dearth of gas
means inner planets remain largely rocky, with thin atmospheres.

This growth process, known as core accretion, proceeds faster
in the outer parts of the disk, where it is cold enough for water to
freeze. The ice beyond this "snowline" supplements the dust,
allowing protoplanets to consolidate more quickly.

They build up a solid core five to 10
times the mass of Earth - quickly enough that the disk remains
gas-rich and the core can pull in a thick atmosphere, producing a
gas giant like Jupiter.

(One of the goals of NASA's
Juno spacecraft, which arrived at
Jupiter earlier this month, is to see whether the planet really does
have a massive core.)

This scenario naturally produces a planetary system just like our
own:

small, rocky planets with thin
atmospheres close to the star, a Jupiter-like gas giant just
beyond the snowline, and the other giants getting progressively
smaller at greater distances because they move more slowly
through their orbits and take longer to hoover up material.

All the planets remain roughly where
they formed, in circular orbits in the same plane. Nice and tidy.

But the discovery of
hot Jupiters suggested something
was seriously amiss with the theory. A planet with an orbit measured
in days travels an extremely short distance around the star, which
limits the amount of material it can scoop up as it forms. It seemed
inconceivable that a gas giant could have formed in such a location.

The inevitable conclusion was that it
must have formed farther out and moved in.

Theorists have come up with two possible mechanisms for shuffling
the planetary deck.

The first, known as migration,
requires there to be plenty of material left in the disk
after the giant planet has formed.

The planet's gravity distorts
the disk, creating areas of higher density, which, in turn,
exert a gravitational "drag" on the planet, causing it to
gradually drift inward toward the star.

There is supporting evidence for the idea.

Neighboring planets often end up
in a stable, gravitational relationship known as orbital
resonance. This happens when the lengths of their orbits are
in a ratio of small whole numbers. Pluto, for example,
orbits the sun two times for every three orbits of Neptune.

It's highly unlikely that they
just happened to form that way, so they must have drifted
into that position, where they were locked in by the extra
stability.

Migration early in our solar
system's history could account for other oddities, including
the small size of Mars and the sparse, disrupted asteroid
belt.

To explain them, theorists have
invoked a maneuver called
the grand tack, in which
Jupiter originally formed closer to the sun, drifted inward
almost to the orbit of Earth, and then drifted out again to
its current position.

Some modelers find such scenarios unnecessarily complex.

"I do have faith in Occam's
razor," says Greg Laughlin, an astronomer at the
University of California (UC), Santa Cruz.

Greg Laughlin argues that
planets are more likely to form in place and stay put.

He says it's possible for large
planets to form close to their star if protoplanetary disks
contain much more material there than previously believed.

Some movement of planets may
still occur - enough to explain resonances, for example -
but,

"it's a final subtle
adjustment, not a major conveyor belt," Laughlin says.

But others say that there simply
could not be enough material to form close-in planets like
51 Pegasi b and others that are even closer.

"They cannot have formed in
situ," physicist Joshua Winn of the Massachusetts
Institute of Technology in Cambridge declares flatly.

And the sizable fraction of
exoplanets that appear to be in elongated, tilted, or even
backward orbits also seems to imply some kind of planet
shuffling.

For these oddballs, theorists invoke a gravitational melee
rather than a sedate migration. A mass-rich disk could
produce many planets close together, where gravitational
tussles would fling them into the star, into weird orbits,
or out of the system.

Another potential disruptor is a
companion star in an elongated orbit. Most of the time it would be
too far away to have an influence, but occasionally it could swing
in and stir things up.

Or, if the parent star is a member of a
tight-knit stellar cluster, a neighboring star might drift too close
and wreak havoc.

Most super-Earths, thought to be largely
solid rock and metal with modest amounts of gas, follow tighter
orbits than Earth, and often a star has several. The
Kepler-80 system, for example, has
four super-Earths, all with orbits of 9 days or less.

The traditional theory holds that inside
the snowline core accretion is too slow to produce something
so large.

And super-Earths are rarely found in
resonant orbits, suggesting that they haven't migrated, but formed
where they sit.
Researchers are coming up with ways around the problem.

One idea is to speed up accretion,
through a process known as pebble accretion. The gas in a rich disk
exerts a lot of drag on pebble-sized objects. This generally slows
them down, causing them to drift in toward the star.

If they pass a planetesimal along the
way, their slow speed means they can be captured more easily,
boosting accretion.

But faster accretion and a gas-rich disk
raise their own problem:

The super-Earths ought to pull in a
thick atmosphere once they exceed a certain size.

"How do you keep them from
becoming gas giants?" asks astrophysicist Roman Rafikov of
the Institute for Advanced Study in Princeton, New Jersey.

Eugene Chiang, an astronomer at
UC Berkeley, says there is no need to speed up accretion, so long as
the disk is solid-rich and gas-poor.

He says that an inner disk 10 times
denser than the one that formed the solar system could easily
produce one or more super-Earths. Chiang has his super-Earths avoid
collecting too much residual gas by forming in the dying days of the
disk when most of the gas has dissipated.

Some early observations from the Atacama Large
Millimeter/sub-millimeterArray (ALMA),
an international facility nearing completion in northern Chile,
support this proposal.

ALMA can map radio emissions from the
warm dust and gravel in disks. The few it has studied so far seem to
be relatively massive.

But the observations aren't yet a
smoking gun, because ALMA is not yet fully operational and it can
only see the outer parts of disks, not the regions where
super-Earths reside.

"Getting close in, that's the
trick," Chiang says - something that ALMA may perform when all
66 of its antennas are working.

Chiang also has an explanation for
another discovery of Kepler's:

superpuffs, a rare and
equally problematic set of planets that have a smaller mass than
super-Earths but appear huge, with a puffed-up atmosphere making
up 20% of their mass.

Such planets are thought to form in a
gas-rich disk.

But in the inner disk, warm gas would
fight against the planet's weak gravity, so the cold and dense gas
of the outer disk is the more likely womb. Chiang invokes migration
to explain their close orbits - a notion supported by the fact that
superpuffs are often found locked in resonant orbits.

MOST OF THE ATTENTION

MOST OF THE ATTENTION in exoplanet
research has so far focused on the inner parts of planetary systems,
roughly within a distance equivalent to the orbit of Jupiter, for
the simple reason that that's all existing detection methods can
see.

The two main methods,

measuring the wobble of stars
caused by the gravitational tug of an orbiting planet

measuring the periodic dimming
of a star as a planet passes in front,

...both favor big planets in close
orbits.

Imaging the planets themselves is
extremely difficult, because their faint light is all but swamped by
the glare from their star, which can be a billion times brighter.

But by stretching the limits of the world's biggest telescopes,
astronomers have seen a handful of planets directly. And over the
past couple years, two new instruments designed specifically to
image exoplanets have joined the hunt (Science, 21 February 2014, p.
833).

Europe's Spectro-Polarimetric
High-contrast Exoplanet REsearch (SPHERE)
and the U.S.-backed Gemini Planet Imager (GPI)
are attached to big telescopes in Chile and employ sophisticated
masks, called coronagraphs, to block out the light of the star.

Not surprisingly, planets far from their
stars are the easiest targets.

One of the earliest and most astounding systems found by direct
imaging is the one around the star HR 8799, where four
planets range in orbits from beyond that of Saturn out to more than
twice the distance of Neptune.

What's most surprising is that all four
are huge, more than five times the mass of Jupiter.

According to theory, planets in such
distant orbits move so slowly that they should grow at a glacial
rate and top out at masses well short of Jupiter's before the disk
disperses. Yet the planets' nice circular orbits suggest they
weren't flung there from closer to their stars.

Such distant giants lend support to the most radical challenge to
standard theory, in which some planets form not by core accretion,
but by a process called gravitational instability.

This process requires a gas-rich
protoplanetary disk, which breaks up into clumps under its own
gravity. These blobs of gas would collapse over time directly into
giant planets without having to form a solid core first.

Models suggest that the mechanism will
only work in particular circumstances:

The gas has to be cold, it mustn't
be spinning too fast, and the contracting gas must be able to
shed heat efficiently.

ALMA is sensitive to shorter wavelengths
that come from dust grains in the mid-plane of the disk, and its
images of the star HL Tauri
in 2014 and
TW Hydrae this year showed smooth,
symmetrical disks with dark circular "gaps" extending far beyond
Neptune-like orbits (see picture, above).

"It was a tremendous surprise. The
disk was not a mess, but has a nice, regular, beautiful
structure," Rafikov says.

These images, suggestive of planets
sweeping their orbits clean as they grow by core accretion, were a
blow to advocates of gravitational instability.

It's too early to tell what other surprises GPI and SPHERE may find
in the outer reaches of planetary systems. But the region between
those outlying neighborhoods and the close-in domains of hot
Jupiters and super-Earths remains stubbornly out of reach: too close
to the star for direct imaging, too far for indirect techniques
relying on stellar wobbles or dimming.

As a result, it is hard for theorists to
get a full picture of what exoplanetary systems are like.

Next year, NASA will launch its
Transiting Exoplanet Survey Satellite (TESS),
and the following year the European Space Agency (ESA) is
expected to launch the Characterizing Exoplanets Satellite (CHEOPS).

Unlike Kepler, which surveyed a large
number of stars in sparse detail to compile an exoplanetary census,
TESS and CHEOPS will focus on bright, sunlike stars close to Earth,
enabling researchers to explore the mid-orbit terra incognita.

And because the targeted stars are
nearby, ground-based telescopes should be able to assess the mass of
their planets, allowing researchers to calculate the planets'
density, indicating which are rocky or gassy.

The James Webb Space Telescope, due for launch in 2018
(Science, 19 February, p. 804), will go further, analyzing starlight
that passes through an exoplanet's atmosphere to determine its
makeup.

"Composition is an important clue to
formation," Macintosh says.

For example, finding heavier elements in
the atmospheres of super-Earths could suggest that a disk rich in
such elements is needed to form planetary cores fast enough.

And next decade, spacecraft such as
NASA's Wide Field Infrared Survey Telescope and ESA's Planetary
Transits and Oscillations will join the hunt, alongside a new
generation of enormous ground-based telescopes with mirrors 30
meters across or more.

If the past is anything to go by, modelers will have to keep on
their toes.